It can sometimes happen that piano supply houses offer felts for sale
that shouldn't come closer than 100 paces to a piano, let alone be
installed in one, touching strings and other metal parts.

These felts have some chemical residue left in them, from the
production or dying process, perhaps salts or acids.  Sometimes, the
felt can actually taste salty if briefly sucked upon.  This can lead
to premature string corrosion.

Any supply company worth their salt (!) would not offer such a product.
They are obviously increasing profits by buying cheap felt and market-
ing it as a specialty product.

Unfortunately, the really proper way to deal with the problem is to
restring the piano again.

Jurgen Goering